The present invention is directed toward a slag and alloy feeding system for use in electroslag melting furnaces and more particularly to a slag and allow feeding system which is based on the measured weight of the consumable electrode.
The electroslag melting process was first invented, developed and put into full production by R. F. Hopkins in Pittsburgh, Pennsylvania during the period between 1930 and 1960. This process employs a consumable electrode which is immersed in a pool of molten slag supported at the top of the resultant solidifying ingot enclosed within a cold-walled mold or crucible.
Alternating (or sometimes direct) current flows down the consumable electrode through the slag, down the ingot and back to the power supply. Preferably, the current flows back to the power supply in a coaxial manner to the top of the crucible such as shown in co-pending application Ser. No. 616,365, filed Sept. 24, 1975. This current, normally in the range of 1,000 amps per inch of ingot diameter, drops from fifteen to forty volts across the slag (or flux) pool thereby producing hundreds of kilowatts of melting power which consumes the tip of the electrode.
As a result of the foregoing, molten metal droplets form on the immersed electrode tip, detach themselves and fall through the molten flux pool to the ingot which is forming there below. As the metal droplets pass through the flux pool, they undergo chemical refinement. Progressive solidification of the ingot formed by this method leads to the physical isotropy and high yield associated with all consumable electrode processes.
Melting rates in the electroslag process are determined by the solidification characteristics of each alloy. However, as an average and for illustration purposes only, such rates are approximately 25 pounds per hour per inch of ingot diameter. Thus, a 24 inch diameter ingot of alloy steel might have an average melt rate of 600 pounds per hour. If this ingot has a typical height of 96 inches, its weight will be 6 tons and total melting time will, therefore, be approximately twenty hours.
As is known in the art, motion of the head of the electrode is the difference between the rate of burn-off of the electrode tip and the rate of build-up of the ingot being formed there below. In the preceding example, a twenty inch diameter electrode would typically be used and its consumable length would need to be greater than the ingot length in the inverse ratio of the squares of their diameter, assuming, of course, full density for both.
As is known in the art, for optimum thermal efficiency and best ingot surface, the ideal position of the electrode tip which is basically flat, provided the electrode to ingot "fill" or area ratio is kept above 0.6, is just immersed under the top surface of the molten flux pool.
However, molten flux in the slag pool is steadily consumed during melting and reappears as a thin (approximately 1/10 inch) skin on the outside surface of the ingot. The precise thickness of this slag skin, and therefore the precise rate of reduction of the slag pool height, is difficult to predict and is therefore difficult to compensate for exactly.
In the past, granular slag to make up these losses due to the formation of the ingot skin and any alloy additions that are required, have been made using batch or belt type timebased feeders. The accuracy of the addition by these prior art feeders was entirely a function of how close the operator of the system estimated the total melting time, and how closely the melt rate came to being constant. For example, the -ton 24 inch ingot discussed above would start with approximately 6 inches deep or 270 pounds of molten flux in the bottom of the crucible. If the ingot slag skin formed is one-tenth of an inch thick, then nine pounds of slag will be lost from the pool to the ingot skin for every foot of ingot height. This means that the slag pool will diminish by 72 pounds, or 27 percent in depth, by the end of the melt unless make-up slag is added during the melt.
As is also known in the art, normal production melting proceeds at a relatively high melt rate until the ingot is one diameter in height, then tapers off slowly in melt rate as the effect of stool cooling diminishes and then reduces relatively rapidly to the "hot-top" level required for a dense ingot head.
Because of the huge thermal time constants involved in the electroslag process, it is very difficult to relate actual melt rate to time at any given power level. As a result, a time-rate slag or alloy feeder is necessarily quite inaccurate. Normal practice with a time base feeder is to set a discharge rate of so many ounces or grams per minute, based on the total weight of slag or alloy required during the melt. The results may be, per inch of ingot, in error by as much as 50 percent, because time is not directly proportional to the amount of metal being melted when the melting power levels are changing.
The situation is even worse for alloy feeding. Assume, for example, that the 6 ton ingot discussed above arrived at the electroslag furnace at a weight of 12,080 pounds, requiring chemistry correction to add 0.1%, or a total of 12.08 pounds, of a critical alloy ingredient. Using the prior art batch weigh and dump feeder described above (belt weighing feeders are even less accurate) it is consistently possible to deliver, at a chute or vibratory feeder: 12.08 .times. 16/20 .times. 60 = 0.161 ounces/minute for a period of 20 hours, with a weighing accuracy of better than 0.1%. However, if the total melt time precalculation is in error by half an hour, or 2.5%, which frequently happens, the time base feeder either delivers too much alloy, thereby starving the head of the ingot significantly or not enough alloy, thereby starving the whole ingot slightly.
More importantly, as the actual ingot weight builds up at different rates due to changes in melting current, the time based feeder continues to deliver 0.161 onces per minute. It will do this irrespective of the number of ounces actually required by the ingot to alloy it correctly. These built-in errors are largely, but by no means entirely, redistributed by the fly-wheel effect of the volume of molten metal in the head of the ingot.